1. Field of the Invention
The invention relates to solventless, sprayable compositions for producing polyurethane and poly(urea)urethane coatings from an isocyanate-terminated prepolymer or quasiprepolymer, the products of such compositions and a process for producing such products.
2. State of the Art
In the past, polyurethane and poly(urea)urethane coatings formed from an isocyanate-terminated prepolymer have been applied by spraying, utilizing a volatile solvent-containing sprayable part A and a volatile solvet-containing, sprayable part B curing agent. The curing agent usually included a substantial quantity (on the order of 50% to 70% by weight of the ultimate coating) of a volatile organic solvent, such as ethyl acetate. These solvent-containing coatings were not commercially satisfactory for many purposes, because the evaporation of the solvent during the formation of the coating results in the production of holes in the coating and a stretching of the polyurethane material as a result of the empty space previously occupied by the evaporated solvent. The stretching weakens the coating, because of stress concentrations resulting from stretch of the coating material, increasing the likelihood of rupture of the coating or wear through abrasion of the coating, particularly at the areas of stress concentration. Solvent-containing coating systems cause other problems, such as solvent entrapment, causing inner coat delamination.
As used herein, a "solventless" system is one in which all or substantially all of the constituents of the part B remain as a part of the final polyurethane or poly(urea)urethane coating.
All of the systems of this invention are two component systems, in which "part A" primarily contains a polyurethane prepolymer or quasi-prepolymer and "part B" is the curing agent which contains the curing compounds which react with the part A to form a polyurethane or poly(urea)urethane coating.
Polyurethane and poly(urea)urethane coatings may also be formed using a so-called "one-shot" system. One-shot systems do not use a prepolymer or a quasi-prepolymer, but rather involve a reaction of an isocyanate with a suitable blend of polyols and polyamines to form the polyurethane or poly(urea)urethane coating. One-shot systems produce an extremely rapid reaction in the formation of the polyurethane or poly(urea)urethane, and do not lend themselves readily to application by spraying, since a relatively uniform coating of the polyurethane or poly(urea)urethane is extremely difficult to achieve. Coatings formed by one-shot systems also are subject to coating degradation by hydrolysis. Accordingly, one-shot systems are not conventionally used for spray application, but rather are used for injection molding, such as for automobile bumpers. When spraying one-shot systems, it is extremely difficult to control physical properties of the ultimate coating so that they are uniform. One-shot systems have been sprayed, but are not commonly used for spraying because of the difficulties attendant upon their use.
It is particularly desirable to be able to spray apply solventless polyurethane and poly(urea)urethane coatings, because it avoids the expense of making a mold which is attendant upon casting processes and avoids the expensive equipment and physical limitations attendant upon the use of centrifugal casting. Furthermore, when dealing with extremely large objects to be coated, such as fly-ash scrubbers, gasoline storage tanks, hopper bins, and the like, it is desirable to be able to field apply the polyurethane coating, in order to avoid the expense of transporting large and cumbersome objects to a factory for coating applicaton. Also, the very size of some of these objects makes application of coatings by prior art processes expensive and difficult, and sometimes impossible.
It is also desirable to create spray-applied systems for producing polyurethane and poly(urea)urethane coatings with superior chemical and physical properties than prior art coatings.
It is therefore highly desirable to obtain a system for spray application of polyurethane and poly(urea)urethane coatings, comprising a part A prepolymer and a part B curing agent, which does not include a volatile solvent, so that the ultimate coating includes all of the constituents of the prepolymer and the curing agent. It is particularly desirable that all of the major constituents of the curing agent participate chemically, at least to some extent, in the reaction forming the polyurethane or poly(urea)urethane, so that a completely chemically bonded coating is formed, which is less susceptible to chemical degradation than is the case if the curing agent includes significant amounts of non-reactive constituents.